1. Field of the Invention
This invention relates to catalysis. In one aspect, the invention relates to novel phosphomolybdic acid catalysts while in another aspect, the invention relates to the use of these catalysts in the oxidation of methacrolein to methacrylic acid.
2. Description of the Prior Art
The art is replete with various phosphomolybdic acid catalysts useful for the oxidation of acrolein to acrylic acid. Illustrative of this art are U.S. Pat. Nos. 4,101,448, 4,115,441, 4,042,533, 4,166,190 and 3,976,688. However, many of these catalysts contain tungsten and many, if not most, whether they contain tungsten or not, are not suitable for the commercial production of methacrolein to methacrylic acid. Of those catalysts that are suitable, none are entirely so. All are subject to improvement in methacrylic acid selectivity and thermal stability. Examples of catalysts now taught to be suitable for methacrolein oxidation include Belgium Pat. No. 823,897, Great Britian Pat. No. 1,482,686, U.S. Pat. No. 3,976,688 and U.S. Pat. No. 4,017,423. The Belgium patent teaches a phosphomolybdic acid catalyst that can be combined with any number of optional components; the British patent teaches a P-Mo-Cu-V-W catalyst that can also be combined with optional components; U.S. Pat. No. 3,976,688 teaches a catalyst similar to the phosphomolybdic acid catalyst of the Belgium patent but containing rubidium, cesium or potassium; and U.S. Pat. No. 4,017,423 teaches a promoted Rb-Mo-P catalyst.